Steel sheet for press forming

ABSTRACT

A steel sheet for press forming having excellent deep drawability and stretchability, the steel of the sheet having W in an amount of from about 0.007 and up to about 0.15 percent. The steel can also have one element selected from the group consisting of Cr and V in an amount of 0.02 to 0.3 percent. It has less than 0.1% C and 0.01 to 0.50% Mn.

United States Patent Nagashima et al.

[451 Feb. 15, 1972 [54] STEEL SHEET FOR PRESS FORMING [72] Inventors: Shinichi Nagashima; Hiroshi Takechi; Youichi Matsuo; Hiroshi Kato; Mineo Shimizu; Nobuyuki Takahashi, all of Kitakyushu, Japan [73] Assignee: Nippon Steel Corporation, Tokyo, Japan [22] Filed: Aug. 19, 1969 [21] Appl. No.: 858,244

Related US. Application Data [63] Continuation-in-part of Ser. No. 601,373, Dec. 13,

1966, abandoned.

[30] Foreign Application Priority Data Dec. 17, 1965 Japan ..40/77319 [5 2] U.S.Cl ..75/123J,75/126C,75/126E, 148/36 [51] Int. Cl CZZC 39/26, C22c 39/50 [58] FieldoiSearch ..75/123, 126C, 123 J, 126E [56] References Cited UNITED STATES PATENTS 2,266,762 12/1941 Jerabek ..75/123 J Primary Examiner L. Dewayne Rutledge Assistant Examiner.loseph E. Legru Attorney-Wenderoth, Lind & Ponack [57] ABSTRACT A steel sheet for press forming having excellent deep drawability and stretchability, the steel of the sheet having W in an amount of from about 0.007 and up to about 0.15 percent. The steel can also have one element selected from the group consisting of Cr and V in an amount of 0.02 to 0.3 percent. It has less than 0.1% C and 0.01 to 0.50% Mn.

4 Claims, 2 Drawing Figures STEEL SHEET FOR PRESS FORMING This application is a continuation-in-part of our application, Ser. No. 601,373 filed Dec. 13, 1966, now abandoned.

The present invention relates to steel sheet for press forming.

Steel sheet used for automobiles and for other press operations is required to have a good deep drawability, stretchability and strain aging property.

it is generally recognized that the deep drawability is determined by an average plastic strain ratioTshown by the ratio of a width strain and a thickness strain as determined by a tension test. Also,Tis usually represented by the following formula T= r,,+2r,,+rw)/4 wherein r, is the value of r in the rolling direction, r is the value 'of r in the direction of 45 to the rolling direction, and r is the value ofr in the direction of 90 to the rolling direction. The deep drawability is better when the value is larger, and in the usual rimmed steelTis about l.3. v

The stretchability indicates the extent of ductility of a material when the material is deformed under biaxial tension and is generally determined by using a work hardening exponent (n) as a'standard. Also, n shows the extent of work hardening and in practice is obtained by a tensile test of a material. The value correspondsto an index (n) in the case of approximating a stress (5)-strain (e) curve by 8=Ce" (C is a constant) and is given by the average inclination of a log 6 to log 6 curve in a strain range of l0 -20 percent. The stretchability is better when the value is larger and in a usual rimmed steel, the n value is about 0.22. An Erichsen test is usually used as the standard, in which test a punch is inserted into a test blank under the condition that the inflow of the material from an outer flange portion into the deforming portion is restrained and the punch travel until the sample is ruptured is measured. The stretchability is better when the value is larger and in the usual rimmed steel the value is about 10-1 1.

Further, a conical cup test is also employed for showing the combined effect of the deep drawability and the stretchability. The effect is better when the value is smaller and in the usual rimmed steel the valueis about 38 mm.

The strain aging property is as follows. When sheet steel is subjected to a plastic deformation, an uneven pattern called stretcher strain" sometimes appears on the surface of the sheet, which impairs the appearance of the article, and hence in order to avoid the formation of the stretcher strain, a light skin pass rolling is usually conducted. However, when a period of time has passed following the application of the skin pass rolling, the effect of the skin pass rolling is lost and the stretcher strain appears again during forming of the steel sheet. This phenomenon is called strain aging. The extent of the strain aging is determined by, for example, providing a prestrain of 8 percent to a sample, keeping the sample for 1 hour at 100 C., and measuring the changed amount of the yield stress before and after the test (A87). in the usual rimmed steel the value (138 is about 5 kgJsq. mm., and ifthe value is less than 1.5 kg./sq. mm., the steel is considered to be nonaging.

The inventors have found, as a result of investigations made in order to improve the above-mentioned properties of steel sheet, that the properties can be markedly improved by the addition ofa specific element. i

That is, a principle object of this invention is to provide steel sheet having very excellent deep drawability and stretchability by adding a small amount of'W to the steel.

Another object of the present invention is to provide steel sheet having very excellent deep drawability and stretchability by the addition of W and also having a good nonaging property by the addition of another element.

Other objects of the present invention will become apparent from the following specification and the accompanying drawings, in which:

FIG. 1 is a graph showing Tvalue of the steel sheet in the cold rolling direction, in the direction of 45 to the cold rolling direction, and in the direction of 90 to the cold rolling direction; and

FIG. 2 is a graph showing the relation between the amounts of chromium and vanadium in steel sheetcontaining them and Tvalue.

in general, experiments have been conducted with respect to structural steel and the like to improve the wear resistance and heat resistance of the steel by incorporating W therei'n. However, it has never been added to steel sheet for press forming for improving the press formability, particularly, the deep drawability and the stretchability.

The inventors have succeeded in producing a steel sheet for press forming having very excellent press formability by preparing steel containing from about 0.007 to about 0.15 percent by weight of W in the steel manufacturing step in a converter, an open-hearth furnace or an electric furnace or in an ingot making step, subjecting it to hot rolling and cold rolling, and then annealing the steel at a temperature higher than the recrystallization temperature, but lower than the Ac transformation temperature.

It is known that the strain aging property in influenced by C and especially N present as a solid solution in steel sheet. Accordingly, if the N present as a solid solution is fixed as a nitride, the strain aging property of the steel can be reduced or eliminated. With respect to W-containing steel sheet, the addition of W only is not effective to give a nonaging property to the steel sheet since W has a poor affinity for N.-.'Therefore, the addition of Al, Ti, Zr, Cr, V, B, etc., which has a high affinity for N may be considered. In the present invention, by the addition of from about 0.007 up to about 0.15 percent by weight of W and 0.02 to 0.3 percent by weight of Cr (and/or V), a good nonaging property can be obtained together with an excellent deep drawability and stretchability. Moreover, in the present invention, a small amount of Cu, P and like may also be added if necessary in a range such that the effect of W is not injured by the addition thereof.

The chemical compositions of the test steels containing various amounts of tungsten are shown in Table 1 together with the chemical composition of a control rimmed steel.

TABLE 1- Chemical composition, weight percent Sample Number 0 Si Mn P S W 0.32 0.014 0.013 Standard. 0. 32 0. 010 0. 009 0. 002 0. 32 0. 011 0. 010 0. 005 0. 28 0.012 0. 010 0. 0076 0. 33 0. 009 0. 012 0. 010 0. 39 0. 0 10 0. 00B 0. 014 0. 36 0. 010 0. 010 0. 021 0.28 0.009 0. 010 0.032 0. 35 0.010 0.009 0.051 0. 28 0. 011 0. 010 0.083 0. 35 0. 010 0.009 0. 0. 32 0. 009 0. 009 0. 152 0. 29 0. 010 0. 007 0. 312 0.31 0.008 0.010 0.520 0. 28 0. 010 0. 009 0. 823 0. 30 0. 003 0. 010 0. 015 0. 37 0. 010 0.011 0.017 0. 38 0. 008 0. 009 0. 031 0. 27 0. 009 0. 010 0. 050 0. 34 0.011 0.007 0.112 0. 29 0. 000 0.008 0. 323 0.28 0. 010 0. 008 0. 524

REMARK9X= Out of the range of the presentlnvention.

The test lsample was melted in a vacuum or in the atsteel h e thqtew wfw i wash u in Table TABLE 2 AS'IM Average Work grain plastic Conical Hardening Sample size strain on exponent Erichsen Number number ratio (r) vs as (n value TABLE 2 AS TM Average Work grain plastic Conical Hardening Sample size strain cup exponent Erlchseh N umber number ratio (1') value n) value X 3 8 0.20 11.0 4. 8 0.30 11.0 5. 8 0. 30 11.0 6 8 0.32 11. 4 7 8 2. 18 0. 31 11. 2 8 8 2.02 0.32 11. 3 9 1. 90 0. 28 10.0 1 9 1.76 0.27 10. 9 11 9 1. 60 0. 26 10. 9 12 9 1. 68 0. 25 10. 8 X 1.42 0. 24 10. 7 X 14- 10 1.17 0.22 10.5 X 1 10 1.08 39.0 0.20 10.2 16 7 1.99 Drawn in- 0.31 11.3 17 7 2.13 0.33 11.4 18 7 2.17 0.32 11.3 10 8 2. 05 0. 28 10. 0 20 8 1. 81 0. 27 10. 0 X 21 8 1. 32 0. 26 10. 6 X 22 9 1.31 0.22 10.4

in the Conical cup value, drawn-In indicates the case where the measurement of the conical cup value is not feasible, because such a good value, i.e., a value of less than 36, was obtained therefor.

As is clear from the results, by the addition of from about 0.007 up to about 0.15 percent by weight of tungsten according to the present invention, the r value, n value, conical cup value and Erichsen value are greatly improved as compared with those of the standard sample. in particular, these improvements are very great when the amount of tungsten added is 0.007 to 0.15 percent.

in FIG. 1, the r value is shown for r,,, r and r respectively, and as is clear from the figures, in the comparative material (dotted curve 1), the order is r r r but in the materials of the invention (solid curves 5 and 17), the order is approximately r =r r although there may be a difference to some extent. The numerals in FIG. 1 show the sample numbers in Table 1.

As is clear from the example shown in FIG. 1, since in the steel of the present invention the difference of the Tvalue between the directional maximum and minimum values is not so great as in the comparative material (1), the formation of so-called ears during drawing is reduced extremely.

The extent of the formation ofears" (Ar) can be approximately shown by A 0 w/ r45 The difference (Ar) in the usual rimmed steel is about 0.6, whereas the difference in the steel of this invention is 0.1 0.2 in an excellent one. and less than 0.4 even in a worse one. Among them, several examples are shown in 'liable 3.

TABLE 3 Plastic strain ratio Sample Number ro us no r Ar Recently it has been shown that the r value of steel sheet has a strong correlation with the texture (cf., e.g., W. .1. Lankford, et al.; Transaction ofthe A.S.M."; vol. 42, 1,197 (1950), R.

having at the sample surface the crystal plane shown by Miller index (1 l l) is larger and the proportion of the crystal having at the sample plane (rolled plane) the crystal plane shown by Miller index (100) is less, the average r value becomes larger and the steel has an excellent deep drawability. (Cf, about Miller index; C. S. Barrett; Structure of Metals, 8(1952); McGraw-Hill).

in Table 4 are shown the results of measuring the values of index (111) and index (100) of the steel of this invention parallel to the sample plane (rolling plane) by an X-ray integral diffraction intensity in which it is shown that the index (1 l 1) is about two or three times as large as that of the comparative material and the index (100) is about one-third to one-third of that ofthe comparative material, which shows the excellency of the mechanical properties of the steel.

TABLE 4 Crystalline direction of rolling plane Sample Number As shown in the example, the range of the content of W in the steel of this invention is from about 0.007 up to about 0.15 percent, preferably 0.007 to 0.08 percent. 1f the content of W is less than the value, the characteristics of the steel are almost the same as those of the usual rimmed steel containing no W, and the deviation of the analytical value becomes larger, which reduces the effect of the addition thereof. Furthermore, if the content is larger than the upper limit, the bad influences thereof on the mechanical properties, such as tensile strength becomes larger and elongation becomes less, which is undesirable. Accordingly, the most preferred range ofW is 0.007 to 0.08 percent.

Usual soft steel contains 0.030.l percent by weight of C, but by subjecting the steel to vacuum melting or treating it in vacuo, the content of the C can be reduced. it is confirmed that the effect of W is remarkable in steel containing any amount of C.

However, if the content ofC is larger than 0.1 percent, the mechanical properties ofthe steel for workability are reduced.

As mentioned above, the content ofC in the steel ofthis invention may be reduced greatly, for example, to less than 0.01 percent by a vacuum treatment and the like during steel manufacturing, but the content of C can also be reduced or controlled by subjecting the steel sheet to a decarburization annealing after rolling.

Furthermore, the content of Mn in steel has an intimate relation with the content of S with respect to red shortness and usually the amount of Mn added is about 10 times as large as that of S on taking into consideration segregation. On the other hand, since Mn hardens the material and reduces the mechanical properties thereof, there is an upper limit for the amount thereof, and the amount thereof in conventional soft steel is about 0.25-0.50 percent.

Howeverfas shown in Table 5, even in the sample, in which the content of S is lower from the first and the amount of Mn added is less than that in the above-mentioned conventional mild steel, the effect of W appears clearly and the objects of the present invention can be achieved. That is, the effect ofW L. Whiteley et al.; Sheet Metal lnd."; vol. 38, 349 (1961), is not reduced even where the steel contains 0.0l-0.50 peretc.), and it is well known that if the proportion ofthe crystals cent of Mn.

TABLE 6 Chemical composition, weight percent STM Mechanical properties grain Conical work- I size cup hardening Erichsen C Si Mn P S W number r value value exponent value J 0. 05 0. 008 0. 05 0. 008 0. 005 7 1. 40 37.9 0. 23 10. 4 0. 04 0. 008 0. 04 0. 009 0. 005 0. 020 7 2. 20 Drawn-in. 0. 32 11. 0.003 0.009 0. 04 0.009 0. 004 0.031 6 2.15 -...do 0.31 11. 5 0. 006 0. 008 0. 06 0. 010 0. 006 0. 083 8 1. 36.8 0. 27 11. 1

The chemical compolsltons and the results in the case of adding W together with Cr and/or V for improving the strain aging property are shown in Tab TABLE 6 Chemical composition, Weight percent Work A6 hardening Mn P S W Cr V kg./mm. 3 r value exponent The procedures for treating the samples in the above case are the same as the above-described procedures. except that the melting is conducted at an atmospheric pressure and the final annealing is conducted in an argon atmosphere.

From the results it was confirmed that although theTvalue and the work hardening component (n) of the steel were reduced by the addition of Cr and V together with W, the strain aging property thereof was greatly improved. However, even though theTvalue and the work hardening exponent (n) may be reduced by the addition thereof, it is clear that they are considerably better than those of the conventional sheetmade of rimmed steel.

On the contrary, when the content of Cr of V was kept constant (about 0.20 percent) and the content ofW was changed, it was observed that theTvalue changed as in FIG. 2. The? value is lower than that of the case of adding W alone, but the value is considerably better than that of conventional rimmed steel.

Mild steel is usually hot rolled in a strip mill, cold rolled and then annealed to be formed into an article. it has been found that the steel of this invention has an advantage in that when it is subjected to similar treatments, the finishing temperature need not be higher than the transformation temperature (Ar This is contrary to the usual case, in which in order to obtain excellent press formability of the final product, it is desirable that the finishing temperature of rolling in conventional hot rolling be higher than the transformation temperature (Ar Furthermore, the cold rolling reduction of the steel of this invention is preferably 40-85 percent and in particular, most preferably 60-80 percent. The annealing after cold rolling is conducted at a temperature higher than the recrystallization temperature, but lower than the transformation temperature (Ac and it is effectively carried out in an inert atmosphere or in a carburizing atmosphere. The aforesaid facts show that the steel of this invention is steel sheet having an excellent press formability.

What is claimed is:

1. Steel sheet for press forming having excellent deep drawability and stretchability, the steel of said sheet consisting essentially of C in an amount less than 0.l percent by weight, Mn in an amount of from 0.1-0.50 percent by weight, W in an amount of from 0.007 percent by weight to 0.15 percent by weight, and the rest Fe and impurities.

2. Steel sheet as claimed in claim 1 wherein the content of C is 0.00l0.l percent by weight, the content of Mn being more than 10 times the content of S, and the content of W being less than 0.15 percent by weight.

3. Steel sheet as claimed in claim 1 wherein the content of W is from 0.007 to 0.08 percent by weight.

4. Steel sheet for press forming having excellent deep drawability and stretchability, and having an excellent nonaging property, the steel of said sheet consisting essentially of C in an amount less than 0.] percent by wei ht, Mn in an amount of from 0.0l0.5 percent by weight, in an amount of from 0.007 percent by weight to 0.15 percent by weight, and 0.020.3 percent by weight of at least one element and im wi ss UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,642,468 Dated February 15, 1972 Inventor) ShlnlChl NAGASHIMA, Hiroshi TAKECHI, Youichi MATSUO Hiroshi KATO, Mineo SHIMIZU and Nobuyuki TAKAHASHI It is certified that error appears in the above-identified and that said Letters Patent are hereby corrected as shown below:

patent Column 4, line 13, change "one-third" to one-half Signed. and sealed this 29th day of August 1972.

(SEAL) Attes t:

EDWARD M.FLETCHER,JR. 7 ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM PC4050 (1 9) USCOMM-DC 6O376-P69 U.S. GOVERNMENT PRINTING OFFICE I959 0-36$-334 

2. Steel sheet as claimed in claim 1 wherein the content of C is 0.001- 0.1 percent by weight, the content of Mn being more than 10 times the content of S, and the content of W being less than 0.15 percent by weight.
 3. Steel sheet as claimed in claim 1 wherein the content of W is from 0.007 to 0.08 percent by weight.
 4. Steel sheet for press forming having excellent deep drawability and stretchability, and having an excellent nonaging property, the steel of said sheet consisting essentially of C in an amount less than 0.1 percent by weight, Mn in an amount of from 0.01- 0.5 percent by weight, W in an amount of from 0.007 percent by weight to 0.15 percent by weight, and 0.02- 0.3 percent by weight of at least one element selected from the group consisting of Cr and V, and the rest Fe and impurities. 